Aluminum is a widely used base metal for various components and metal pieces because of it's relatively low weight and high corrosion resistance, However, aluminum in a pure state is a relatively soft metal with a yield strength of only 34.5 N/mm.sup.2 and a tensile strength of 90 N/mm.sup.2. The relative softness of aluminum may be overcome by using a suitable alloying material and by heat treatment. A large number of alloys having a range of strength and ductility may be achieved using various known alloying elements, and using appropriate concentration of those alloying elements. Some common alloying elements added to aluminum are copper, magnesium, silicon, manganese, nickel and zinc. Each of these may be used to increase the strength and/or the casting properties of pure aluminum.
It is well known that many metallic components, such as a brake piston, e.g., are given a surface treatment such as anodizing prior to the component being used. The surface treatment is intended to increase the functionality and lifetime of the component by, for example, improving one or more of heat resistance, hardness, electrical conductivity, lubricity, or the cosmetic value of the component.
One known method of anodizing aluminum (forming an oxide film on the aluminum) entails subjecting the aluminum to an acid electrolyte, often composed of sulfuric acid or an electrolyte mixed with sulfuric and oxalic acid. The anodizing process is typically performed in electrolytes containing 12-15% v/v (by volume) sulfuric acid at a low process temperature (between -5.degree. C. and +5.degree. C. e.g.). Higher concentrations and higher temperature decrease the oxide formation rate significantly. Also, higher concentrations and higher temperatures decrease the formation voltage, which adversely affects the compactness and the technical properties of the oxide film.
Electric current is provided through the electrolyte and aluminum component to cause the film to form. Typical prior art power supplies used for the conversion of metallic aluminum into a ceramic coating (aluminum oxide or alumina) provide a direct current having a density of typically between 3 and 4 A/dm.sup.2. The anodization is carried out at a relatively low temperature and fairly high current density to increase the compactness and technical quality of the coating performance (higher hardness and wear resistance).
The anodization produces a significant amount of heat. Some heat is the result of the exothermic nature of the anodizing of aluminum. However, the majority of the heat is generated by the resistance of the aluminum towards anodizing. Typically, the reaction polarization is high, such as from 15-30 volts, depending upon the composition of the alloying elements and the process conditions. Given typical current densities, from 80% to 95% of the total heat production will be resistive heat.
Prior art processes for anodizing aluminum attempt balance the electrolytic conversion of aluminum into aluminum oxide and the chemical dissolution of the formed aluminum oxide because of the acidic nature of the electrolyte. The total production of heat is a significant factor influencing the desirable balance and determines the final quality of the anodic coating. Heat must be dispersed from areas of production toward the bulk solution at an efficient rate. Heat produced at the aluminum surface is dispersed in conventional anodization by air agitation or mechanically stirring of the electrolyte in which the oxidation of aluminum is taking place.
If the balance between formation and dissolution is not properly struck, and dissolution is favored, the oxide layer may develop holes, exposing the alloy to the electrolyte. This often happens in prior art anodization methods and is known as a "burning phenomena".
It is desirable to make castings that have a sound structure free from porosity, entrained oxides, and segregation effects. However, the type of alloys which are easiest to cast such as high silicone alloys (7% or more) and high copper alloys (2% or more e.g.) are least suitable for anodizing. Due to the high content of copper, it is difficult to obtain thick oxide coatings without the occurrence of burning.
A typical prior art galvanostatic (i.e. current controlled) anodizing process uses direct current until a bath voltage of approximately 30 volts, depending upon the anodization conditions such as sulfuric acid concentration, process temperature, anodizing current density, etc., is reached. After the bath voltage reaches 30 volts the voltage is increased step-wise until the bath voltage reaches approximately 40 volts.
Generally, burning may be prevented if the voltage changes from 30 to 40 volts do not result in current increases. Thus, prior art methods use a step-wise increase in voltage that is determined by the current response from the anodizing tank. The user monitors the current and increases the voltage in small steps to insure that the current doesn't change. The step wise voltage increases are typically performed over a relatively long period of time, such as 20 minutes or longer, in order to avoid burning. The change from 30 to 40 volts must be performed very carefully because that is a critical period where burning is very likely to occur.
When the bath voltage reaches approximately 40 volts, anodizing is continued with constant current until a specified thickness is reached. Using this prior art technique it is very difficult from batch to batch to obtain identical conditions, and the reject rate caused by burning is typically about 10%.
Another prior art anodizing uses square wave current pulses. Pulses are used to provide periods of time during which the oxide is formed and periods of time during which heat is dispersed (i.e. rest periods). One prior art current pulse pattern uses a square wave having a first higher current magnitude for oxide formation, followed by a second lower (close to zero) current magnitude. The relative durations of the higher magnitude and lower magnitude currents determine the relative amount of oxide formation and heat dispersion. One such type of simple pulse pattern may be found in U.S. Pat. No. 3,857,766 or Anodic Oxidation of Al. Utilizing Current Recovery Effect, Yokoyama, et al. Plating and Surface Finishing,1982, 69 No. 7, 62-65. This type of current pulse pattern is shown in FIG. 1.
U.S. Pat. No. 3,983,014, entitled Anodizing Means And Techniques, issued Sep. 28, 1976 to Newman et al., discloses another type of pulse pattern. The pulse pattern described in Newman has a high positive current portion, followed by a zero current portion, followed by a low negative current portion, followed again by a zero current portion. Each of the pulse portions represent one quarter of the cycle. Thus, the current has a high positive value during the first quarter of the cycle. No current is provided during the next quarter of the cycle. The current has a low negative value during the third quarter cycle. Zero current is provided during the final quarter of the cycle.
Another prior art pulse pattern is described in U.S. Pat. No. 4,517,059, issued May 14, 1985, to Loch et al. Loch discloses a pulse pattern that is a square wave alternating between a relatively high positive current and a relatively low negative current. The durations of the positive and negative portions of the pulses are controlled used in an attempt to control the anodizing process.
U.S. Pat. No. 4,414,077, issued Nov. 8, 1983, to Yoshida et al. describes a train of pulses superimposed on a dc current. The pulses are of a plurality opposite to that of the dc current.
Other prior art methods use a sinusoidal voltage wave, or portions thereof, applied to the voltage buses used for generating the anodizing currents (i.e. potentiostatic pulses). However, such prior art systems do not utilize current pulses for controlling the anodizing process. Examples of such prior art systems may be found in U.S. Pat. No. 4,152,221, entitled Anodizing Method, issued May 1, 1979, to Schaedel; U.S. Pat. No. 4,046,649, entitled Forward-Reverse Pulse Cycling Pulse Anodizing And Electroplating Process issued Sep. 6, 1977, to Elco et al; and U.S. Pat. No. 3,975,254, entitled Forward-Reverse Pulse Cycling Anodizing And Electroplating Process Power Supply, issued Aug. 17, 1976, to Elco et al.
Each of the aforementioned prior art methods, while utilizing a pulse of some sort, does not provide adequate hardness and thickness while maintaining a low reject rate. Moreover, such prior art systems are relatively slow and take a relatively long period of time to complete the anodizing process. These problems are particularly found when the prior art methods are used to anodize aluminum alloys containing high concentrations of alloying copper, with or without silicon as a second alloying element.
Accordingly, it is desirable to provide a method of anodizing an aluminum alloy part at a fast rate without destroying the oxide film caused by burning phenomena, and without lessening the functional improvements provided by anodizing. Additionally, such a method should lessen the production cost and be particularly suitable for forming thick oxide films on aluminum alloys containing high concentrations of alloying copper (2% or more), with or without silicon as a second alloying element.